Best or fastest finish - gloss or matt?

hi
I am restoring an old Tiga race long board, 3.6m of retro!

ive filled a few dings and painted a two pack polyurethane finish coat ( PPC Whitecoat) to seal up the epoxy repairs & seal the board edges where the two plastic halves were joined. This paint leaves a high gloss surface that is very very tough.

the original finish was a semi gloss plastic, so my mind was wondering what's considered the fastest surface these days?
I have a FW 167 that has a matt finish.

i would like to, once the paint has fully dried, sand it back to remove the painting imperfections, so it will then be mstt, but I could then buff it to gloss again. ( same as buffing an auto finish).

So so what's the quickest?
thanks
nigel

good luck mark,

let me know if you want my flame suit.

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Mark _australia said..
Fastest - always matte

rub back to about 600 or 800 wet.

had a look at the bottom of any new Mistral Speed or Slalom board lately Mark? Shiny as.....

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sick_em_rex said..

Mark _australia said..
Fastest - always matte

rub back to about 600 or 800 wet.

had a look at the bottom of any new Mistral Speed or Slalom board lately Mark? Shiny as.....

some degree of matte-ness is always preferable until certain speed is reached (dunno what that is though)



you can see smooth has a higher co-efficient of drag early on. As to whether this makes a 0.001kn difference, or a more useful 0.5kn difference, I dunno.

If a certain board is faster, it is more likely to be design / shape. Especially as the coefficient of drag changes so greatly under different conditions. It is a pointless argument really.

It'd be great to hear from someone with a level of expertise in fluid dynamics (Slowboat et al) to confirm or disprove that once planing the drag coefficient of a board's surface would be minimal compared to the drag coefficient of the fin i.e. the fin plays a much greater part in going fast than the board's surface therefore it doesn't matter if it's polished or sanded?????

The above scenario is for a planing hull whereas a longboard when not planing would be affected more by the hull's drag coefficient so surface finish may be more critical????

Hey Barn, are you saying I'm a clown or an overly religious zealot

I don't know about the boards surface, but while re-reading the old thread this post from GAZ stood out for me:

"looks like this post has been stagnant for a bit but I recently came across an interesting finology article on vmgblades.com (excerpt below) and couldn't help but wonder whether principles of laminar and turbulent flow apply somewhat the same to the bottom of boards as they do to fins? Finish & Flow (of fins) Finish is how the surface of the fin feels and has a large part in how the water flows over the foil. This is why looking after your fin is very important. Nicks, scratches or dents all affect the flow and can create drag or separation bubbles. If you have a tendency to spin out unexpectedly, then this is the section for you to read. Laminar flow is where the water stays attached to the fin and follows the foil shape closely. Theoretically this creates the least amount of drag (always a good thing as we want to minimise drag). However, laminar flow can stall dramatically and you find it much easier to spin out. This is because large separation bubbles happen in the boundary layer of the water flow around the foil. The separation bubbles separates the fin from the water, you suddenly have no lift and you spin out. Turbulent flow is like little swirls of turbulence along the fin. More energy is created in the boundary layer and the separation bubbles are tiny. As the bubbles are tiny, stalling is less likely to occur and a rider is able to push against the fin as it is more stall resistant. This gives you more control and allows you to be more daring in your tactics. There is slightly more drag but at least you don’t have to worry about spinning out in the water. A rougher finish forces a fin into making turbulent flow and a smoother fin is more likely to become laminar flow. Sailors often experience different sensations when sailing new fins. This is because of their style and experience. It can also take a little while to get used to a new fin and adapt your style. Different finishes could be recommended to different sailors, this is so that you get the best possible ride for your style."

Around 2008 - 2010 Mal Wright and I were experimenting with very fine fin finishing. Our usual routine the evening before a big day was to spray our fins with 3d spray paint and sit around for hours very carefully wet bock sanding with fine W&D, and then polishing the fins with wax/cutting compound to a mirror finish.

I don't know if it gave us a speed advantage, and if it did, whether the benefit was due to immaculately smooth fins from the prep or from the polishing. I do know we were consistently doing some pretty good speeds in less than manic wind conditions back then.

But the paragraph highlighted above is very interesting. One thing I definitely noticed was that the highly polished fins had a tendency to be a bit "squirrelly" ("technical" term that means they sometimes kinda half let go when accelerating under pressure) for the first few runs! I wonder if that was linked to the info in the above statement?
Often the first few runs were also the fastest. Could it be that we were getting a benefit from 'laminar flow' over the fins resulting in less drag?
The highly polished finish seemed to dull after a few runs, probably due to sand abrasion in my case! and those funny little brief 'squirrels' would disappear.

These days I don't pay anywhere near as much attention to fine tuning my fins, but after LG this year I have to say that Sammy the Snail has inspired me again!

It's the other way around. If you look at the stance of someone riding a windsurfer and do the vector equilibriums you come to the conclusion that the vertical lift required of the board is at least twice the lateral lift required of the fin. Then you have to look at the lift to drag of a hull vs a fin. The fact that foiling windsurfers are slipperier than planing windsurfers demonstrates that fully submerged fins are less draggy.

The hull of a planing windsurfer thus loses out both ways. It's got twice as much work to do and it's doing it harder. That's where most of the drag is.

Frank Bethwaite did a lot of measurements of foil and hull performance vs. surface finish. He was very thorough, he went as far as getting a foil chrome plated! He also goes with the polished surface being best.

On the other hand, the theory goes that if the imperfections are well within the viscous sub-layer then it shouldn't make any difference. The scratches from 600 grit sandpaper sit well within the viscous sub-layer. A quick going over with 600 grit sandpaper at least ensures there are no nicks, scratches, squashed insects or paint bubbles that might be poking through to negate the gloss finish.


en.wikipedia.org/wiki/Law_of_the_wall

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sailquik said..
Interesting Ian!

Do you know what hull speeds Braithwaite tested? Displacement or Planing?

He shows drag tests plotted at various trim and heel angles at speeds up to 12 knots, planing for small dinghies. He detected differences in drag in boats that had collected barely visible road grime, but he basically seems to have inferred the best finish from his testing of foils. Even his testing of foils was a little indirect, he measured the maximum lift that could be generated at each speed as a function of finish. An indirect method of establishing the best finish for lift to drag I suppose. But so much useful stuff in this book, " Frank Bethwaite, High Performance Sailing 1996". He even covers sailboards, a great shot from P259.

And there is a good point.
For the "gloss is faster" camp: How many imperfections, or how much orange peel effect is present in your glossy board?

Block sand it briefly with say 1200 wet n dry and you will see circles all over it cos it aint flat. Anyone who has painted a car will be well and truly familiar with that. It is hard to see orange peel in white, especially in the sun.
Sanding will make it all even, and even is better (less turbulence)

So if you feel gloss is better you should rub back you factory board with 600 - 800 - 1200 - 2000 - 2500 (wet) then polish well.

If you feel matte is better you do the above and stop at 800.

Personally, like I said I feel matte is better, but the best research shows it is negligible so who cares.

OTOH god knows how I got redthumbed for the graph above which shows the coefficient of drag is higher for smooth surfaces over a wider range of conditions. Maybe we should just rely on gut feeling and the fact a Mistral is fast.

I wet sanded a couple of my boards that had thick graphics on the bottom. Flattened them out nicely and got the whole tail area very smooth. Went to 1000 which puts a slight sheen on the board.

God knows if it had any benefit, but it made me feel good.

Crickey!!
didnt realise this would happen, almost as good as asking what oil to put in a 928!!

anyway I flat sanded the surface as there were many imperfections from painting & regardless they are now nearly gone - some more to be done there.
Just like wet sanding a car to remove orange peel

thsnks again , nigel

^^^ use fish oil then

If you are racing one stuffed up gybe and all this amounts to nothing

www.fsc.com.au

This ones mine ! Carnuba Wax

You lost me at the first line:

"If you get a group of 5 boaters to sit around the table with a jug of beer ..."

...You're going to have two empty glasses.